Thermal energy storage compositions

ABSTRACT

Thermal energy storage compositions comprise a thermal energy storage material, for instance an inorganic hydrate that can undergo a change of hydration with accompanying release or storage of latent heat of hydration at a temperature of 10° to 100° C., held in a matrix of polymeric material, generally acrylic polymer. In one aspect of the invention the composition is in particulate form and so can, for instance, be poured into the heat exchanger in which it is to be used. In another aspect of the invention the storage material includes polyvalent metal ions and the polymeric material is a cationic polymer. In another aspect of the invention the composition is made by forming a solution, generally in water, of all the polymerizable material and most or all of the storage material and then polymerizing the polymerizable material in this solution. The solution is generally a low viscosity fluid and polymerization may be conducted in the heat exchanger vessel in which the composition is to be used.

This application is a division of Ser. No. 465,379, filed Feb. 10, 1983,now U.S. Pat. No. 4,470,917.

There have been numerous proposals for storing thermal energy by makinguse of the latent heat of thermal energy storage materials. A thermalenergy storage material is a compound, or mixture of compounds, thatwill reversably undergo a modification or change of state withaccompanying release or storage of latent heat, and optionally includesalso one or more promotors, such as nucleating agents, for the change ofstate. One suitable reversable change of state may involve fusion andsolidifcation, in which event the material will absorb thermal energywhile it is being converted from the solid state to the fused state butwill release thermal energy when it converts from the fused state to thesolid state. Another suitable reversable change of state may involve achange in the degree of hydration of a hydrate.

Such thermal energy storage materials have great potential value, forinstance as a means of storing heat collected by a solar energycollector and releasing it when solar energy is not available.Unfortunately their use involves a number of serious problems.

One problem is that it is often difficult to achieve satisfactorycontact, in a convenient manner, between the thermal energy storagematerial and a heat exchange fluid that is to transport thermal energyto or from the material. For instance efficient direct contact betweenheat exchange fluid and the thermal energy storage material has not beenpracticable and indirect contact requires that the storage material beheld in containers of appropriate shape, and it can be difficult to fillthe containers satisfactorily with suitable material. Another problem isthat the thermal energy storage material, in one of its states, isgenerally heterogeneous, consisting of a mixture of a liquid phase witha small amount of a solid phase. This solid phase may be provided bynucleating agent or by material that is formed during incongruent fusionof the thermal energy storage material. There is then a tendency for thesolid phase to separate from the liquid phase and this will reduce theefficiency with which the liquid phase can subsequently be returned tothe solid state.

To minimise the separation of the solid and liquid phases it is known toprovide a thermal energy storage composition in which the storagematerial is dispersed in a viscous liquid or other thickenedcomposition. Such thickened compositions are discussed in, for example,U.S. Pat. No. 3,986,969. As explained in that patent the resultantcompositions were often not entirely satisfactory, for instance becausethe thickening action of the thickening agent tends to degrade duringuse, and also the thickening systems discussed in that patent tend to beincapable of preventing separation of solid phase present in fusedthermal energy storage material.

In an attempt at overcoming these difficulties it is proposed in BritishPat. No. 1,584,559 and U.S. Pat. No. 4,273,667 to mix the storagematerial with a polymer, which in practice is a linear acrylic polymer,and then to cross-link the polymer by particular mechanisms described inthose patents, so as to form a hydrogel, and then to introduce thehydrogel into the heat exchanger in which it is to be used.

One difficulty with all these systems is that the heat exchangers inwhich the composition is to be used are generally of complex shape, forinstance thin panels, in order to facilitate indirect heat exchangebetween the heat exchange fluid that is to transmit thermal energy to orfrom the storage material. As a result of these complex shapes it can bedifficult to fill the composition satisfactorily into the heat exchangevessels and this difficulty increases when the composition is in theform of a highly viscous liquid or a gel. Thus attempts at reducing theseparation of solid and liquid phase by increasing the viscosity of theliquid, such as in British Pat. No. 1,584,559 and U.S. Pat. No.4,273,667 increase the difficulty of filling the storage systemsatisfactorily into the vessel and in these two patents the mixture evenbefore cross-linking is so viscous that it is difficult to fill it intosome heat exchange vessels.

Another difficulty is that uniform blending of the storage material witha viscous liquid can be difficult. Blending with linear polymers cantherefore be difficult and the use of previously cross-linkedpolyacrylates leads to serious blending difficulties. Particulardifficulties can arise as a result of blending acrylic acid polymerswith storage material such as calcium chloride in that it can causeuncontrolled cross-linking and insolubilisation during the blending andthe resultant blended product may become even less homogeneous duringstorage.

Storage structures are proposed in U.S. Pat. No. 4,003,426 that comprisea matrix, in an amount of at least 25%, of a cross-linked polymericresinous material selected from polyesters, polyvinyl esters andepoxides, and that includes cavities containing heat storage material.The product is made by dispersing the storage material in the liquidmonomer and then polymerising the monomer. Although inorganic hydratestorage materials are mentioned it is difficult to obtain uniformdispersion of the heat storage material in the polymeric matrix sincethe storage material has to be dispersed in liquid monomer, which isthen polymerised.

An entirely different approach is described in British PatentSpecification No. 2031936A, in that storage material in fine particulateform is dispersed in an oil containing an emulsifying agent. Howeverthis also is not entirely satisfactory. For instance, there may beseparation of either the entire thermal energy storage material or ofthe solid material of a heterogeneous mixture that may be formed duringuse of the thermal energy storage material.

It has been our object to provide new types of thermal energy storagecomposition that avoid or minimise the disadvantages of the knowncompositions, and to provide improved methods of making thermal energystorage compositions.

A thermal energy storage composition according to one aspect of theinvention is particulate and comprises a thermal energy storage materialheld in particles of a solid, thermally conducting, matrix of polymericmaterial.

The particulate composition may be poured into a container of any chosenshape and may be subjected to direct or indirect contact with a chosenheat exchange fluid.

The solid form of the matrix may be required to persist only until theparticulate composition is positioned in the chosen container,whereafter the particles may flow into one another, for instance whenthe thermal energy storage material is fused. Preferably however thematrix is substantially solid throughout use so that the particlespreferably retain their identity and original shape during use orpossibly agglomerate to some extent but without fusing together to givea solid mass. However it is also possible, but less preferred, for theparticles to fuse during use into a solid mass provided this does notinvolve the matrix becoming so fluid that it permits separation of thestorage material. Since the matrix is a solid, and does not becomeliquid during normal use, there is substantially no risk of phaseseparation of components of the thermal energy storage material duringuse.

The matrix may be a relatively rigid solid, the particles then alsobeing relatively rigid. However the matrix may be a soft or flexiblesolid, and may have a texture such that the matrix of each particle canbe described as a non-flowable gel. The particles may then be hard whenthe thermal energy storage material is solid but soft and rubbery whenit is fluid, for instance above its transition temperature.

The size of the particles may be chosen from a wide range. For instancethe particles may have a particle size as small as 1 micron or as largeas 10 cm. The optimum size will be selected having regard to theintended use of the particles.

Thermal energy may be transmitted to or extracted from the heat storagematerial by direct contact with heat exchange fluid. The fluid must bechemically inert to the composition in order that it retains its shapeduring use, and so usually the fluid is non aqueous. For instance inertheat exchange fluid may percolate through a bed of particles of thecomposition. It is then generally desirable that the particles shouldnot be too small, as otherwise there may be a risk of them being carriedaway with the heat exchange fluid. The particle size will be at least0.01 cm and preferably at least 0.1 cm. Normally it will be below 5 cm,preferably below 2 cm. Particles in the range 0.5 to 2 cm areparticularly preferred. Normally the particles have similar dimensionsin each direction, for instance being spheres, cubes or randomly shapedparticles, but if desired they may be, for instance, rod shaped.

The composition can also be used for indirect heat exchange, beingfitted in an indirect heat exchanger that may be of conventional shape.In order to ensure good heat exchange between particulate compositionand the heat exchange fluid it is desirable to pack the particlestightly in the heat exchanger, so as to minimise the void space betweenadjacent particles. Thus the shape and size of the particles ispreferably such that the void space between adjacent particles is lessthan 50%, and preferably less than 30% (typically about 20%) by volumebased on the volume of particles and the void space. It is alsopossible, but less preferable, to fill the void space with a heatexchange fluid that may be static in the vessel, satisfactory resultsthen being obtainable with larger void spaces, e.g. up to 75%, althoughthe smaller void spaces are generally preferred.

In another system it may, however, be desirable to have very fine solidparticles, for instance having a particle size from 1 to 100, usually 1to 20, microns and typically about 4 microns. These very fine particlesmay be used for indirect heat exchange, being poured into a suitableindirect heat exchanger with the void spaces filled by air, or they maybe used in the form of a suspension in a heat exchange oil, which may beused for indirect heat exchange contact with a heat exchange fluid ormay be used as a heat exchange fluid that is pumped around the heatrecovery circuit. The invention includes heat exchangers containing theparticulate composition.

The polymeric material that provides the matrix of the composition maybe formed from a wide variety of polymerisable monomers. The preferredmonomers are ethylenically unsaturated monomers, especially acrylicmonomers. If the thermal energy storage material is to be present as ahydrate it is desirable that the matrix should be capable of absorbingsome water. Suitable acrylic monomers include acrylic and methacrylicacid and their salts, acrylic and methacrylic esters and acrylic andmethacrylic amides, preferred monomers being acrylamide, sodium acrylateand dialkyl amino alkyl(meth) acrylates and dialkylaminoalkyl(meth)acrylamides, including quaternised derivatives of the dialkylaminocompounds, for instance quaternised dimethylaminoethyl acrylate. Thepolymer may be a Mannich base of polyacrylamide. Copolymers of 2 or moreof the described monomers are often preferred.

Other polymers that may be used include polymers (including copolymers)of vinyl pyrrolidinone or vinyl sulphonic acids (that may be polymerisedfrom aqueous solutions as described above) and styrene maleic anhydridecopolymers and polymers formed from dimethylamine and epichlorhydrin.Thermosetting polymers, such as epoxies and polyesters, may be used.

The polymers are often cross-linked in the final particles. Thecross-linking agent will be chosen having regard to the polymer that isto be cross-linked. Suitable cross-linking agents for use with acrylicmonomers include methylene bisacrylamide, methylol acrylamide andsoluble polyethylene glycol diesters. The cross-linking is ofteneffected after formation of the particles of the matrix polymercontaining the storage material.

Broadly any polymeric matrix that can provide a solid thermallyconductive matrix in which the storage material is held may be used, andthis requires that it should be possible both to form a substantiallyuniform or homogeneous blend of the storage material in the polymer andto provide this blend in the form of particles.

It is generally preferred to make the composition by blendingpolymerisable material with the heat storage material and then eitherpolymerise the polymerisable material while in the form of an emulsionor dispersion (so as to form emulsified or dispersed polymer particles)or polymerising the polymerisable material to form the polymeric matrixin bulk form and comminuting the mixture during or after polymerisation.

The polymerisation should be conducted while the thermal energy storagematerial is homogeneously distributed with respect to the polymerisablematerial, and it may be necessary to thicken the system or to agitatethe system in order to minimise phase separation until polymerisationhas occurred. Polymerisation is preferably conducted on a solution inwhich both the polymerisable material and most or all of the storagematerial are dissolved. For instance water soluble monomer and storagematerial that can be in the form of a hydrate may be dissolved in theamount of water required for providing that hydrate. Polymerisation maybe conducted on the solution while it is in bulk or while it isdispersed or emulsified in another liquid.

Instead of mixing the thermal energy storage material with polymerisablemonomers it may be mixed with partially polymerised material, andpolymerisation then completed to form the solid matrix. For instance alinear acrylic polymer may be mixed with the storage material and maythen be cross-linked by reaction with a suitable cross-linking agent,for instance materials mentioned above, aldehydes such as glutaraldehydeor formaldehyde or, when the polymer contains carboxylic or sulphonicacid groups, cations or polyvalent metals.

Polymerisation of acrylic monomers may be by thermal or redox systems,including thermally decomposing initiators such as ammonium persulphate.

If it is desired that the composition be in the form of very fineparticles then they are conveniently made by emulsion or suspensionpolymerisation, for instance inverse polymerisation (i.e. polymerisationof a water-in-oil emulsion or dispersion of water soluble polymerisablemonomer, followed by recovery of the polymer if desired). Alternativelythe polymer can be formed in powder form by other conventional methods.Preferably however the formation of the matrix polymer is conductedwhile the composition is in bulk form and the composition is thencomminuted to the desired particulate form, with polymerisation beingcompleted before or after comminution. Comminution may be achieved by,for example, extruding extrudable composition through an appropriateorifice. The composition may be extrudable as a result of thepolymerisation not having been completed at that stage or, if the matrixis heat softenable, by heating the composition above its softeningpoint.

The matrix must be thermally conducting, by which we mean that thematrix must not prevent conduction of thermal energy into and out of thestorage material within the matrix. Naturally some reduction in heattransmission may have to be tolerated, but it must not be so great as todestroy the effectiveness of the thermal energy storage material. Thematrix is generally of non-foamed polymer and it is then found thatsubstantially all synthetic polymers are capable of providing a suitablythermally conducting matrix. The weight of the matrix, based on thecombined weight of the matrix and the storage material, should be as lowas possible. It is generally below 50% and preferably it is below 25 oreven 20%. Normally it is at least 1%, preferably 5 to 15% by weight.

The thermal energy storage material is, as stated above, a compound ormixture of compounds that will reversably undergo a modification orchange of state with accompanying release or storage of latent heat, andoptionally includes also promotors for the change of state such asnucleating agents. Preferably it is a material that provides, afterstorage of latent heat, a heterogeneous system comprising a liquid phaseand at least one solid phase of a different density from the liquidphase.

The modification or change of state may involve various phenomena. Inits simplest form it will be reversable fusion and solidification. Inanother form it will involve change from one crystalline form toanother. In another form it will involve chemical change, e.g. a changein the degree of hydration of an inorganic hydrate. Thermal energystorage materials that undergo such modifications or changes of stateare well known in the literature and may be used in the invention.Preferably the change of state occurs at a temperature between 10° C.and 100° C., since this is the preferred temperature range for use insolar or other heat energy storage systems and preferably the change ofstate is accompanied by a high release or consumption thermal energy.Typically the latent heat of a suitable material is above 30 andgenerally above 50, and most preferably from 70 to 150, KWh/m³.

Typical thermal energy storage materials include sodium sulphate andcalcium chloride and other inorganic hydrates and others are disclosedin, for instance, the aforementioned British and U.S. patentspecifications. Hydrates must be provided within the matrix togetherwith sufficient water to form the desired hydrate. The particles maycontain this water when formed initially. For instance sodium sulphatedecahydrate or calcium chloride hexahydrate may be provided by combiningthe anhydrous salt in a polymerisable mixture containing sufficientwater to form the desired hydrate within the matrix. Alternatively when,as is preferred, the matrix is water permeable the water content of theparticles may be adjusted after their formation by evaporation or byabsorption, for instance of the theoretical amount of water for formingthe hydrate.

A preferred product of the invention uses a storage material that formsa hydrate in at least one of its phases and that is in an anhydrous formor the form of a low hydrate. Such products are usually more freeflowing than the product obtained on hydration of the product. Thus themore free flowing, less hydrated product may be poured into the chosenheat exchange vessel and hydrated in that.

To promote the desired change of state, and in particular to promotecrystallisation and prevention of supercooling, it is normal to includea nucleating agent. Suitable nucleating agents are discussed in theaforementioned British and U.S. Patent Specifications and in theliterature referred to in those. The amount of nucleating agent istypically from 1 to 20% based on the amount of salt hydrate or othercompound that undergoes the reversible change of state, the remainder ofthe storage material being the compound or compounds that will undergoreversable change of state.

The following are some examples of this aspect of the invention.

EXAMPLE 1

In this example particles of heat storage material within an acrylamidecopolymer matrix are prepared and are then dispersed in oil.Polymerisation is by dispersion polymerisation. A monomer feed isprepared by mixing together

    ______________________________________                                        NaSO.sub.4             248.0  g                                               Borax                  28.1   g                                               H.sub.2 O              314.1  g                                               Acrylamide (52.5%)     34.2   g                                               Glacial acrylic acid   42.0   g                                               NaOH (46.0%)           50.7   g                                               Ammonium persulphate   0.032  g                                               Methylenebisacrylamide 0.954  g                                               ______________________________________                                    

Sufficient additional water (200 g) is then added to ensure completedissolution of all components. The above monomer feed is added dropwiseto a reaction vessel containing the following reagents at 40°-50° C.

    ______________________________________                                        Tetrachloroethylene    400.0  g                                               SBP 11 (paraffin)      70.0   g                                               Stabiliser             12.0   g                                               ______________________________________                                    

Initiation of polymerisation occurs as the vessel contents are heated to80° C. This temperature is maintained for 3/4 hr to 1 hr. Thetemperature is then increased to reflux and water in the azeotropedistillate was separated off. The final water content of the beadsobtained is measured using a Dean and Stark apparatus. If necessary,water is added to rehydrate the beads to the correct water content forcrystallisation.

A sample of beads which has been produced in this manner is dispersed ina hydrocarbon oil and the product filled into an indirect heat exchangerin which it was heated to 45° C. to allow transition. The product isthen allowed to cool, and the expected crystallisation of the hydratedsalt, with accompanying release of heat, is observed. This transitioncan be reversably repeated over many cycles.

EXAMPLE 2

In this example the matrix is formed of cross-linked polyacrylamide bygel polymerisation using a monomer and salt mixture including thermalinitiator, as follows:

    ______________________________________                                        CaCl.sub.2             48.4   g                                               K Cl                   4.5    g                                               SrCl.sub.2 6H.sub.2 O  1.0    g                                               Acrylamide soln (52.5%)                                                                              15.2   g                                               Methylene bisacrylamide                                                                              0.016  g                                               Ammonium persulphate   0.004  g                                               H.sub.2 O              39.9   g                                               ______________________________________                                    

The mixture is heated in a beaker on a hot plate to 75° C., when theviscosity increases and then a gel is formed. This rubbery gel is thenextruded into prills having a volume of 0.05 to 5 cc.

The resultant prills are dehydrated by heating at 110° C. to form a freeflowing product that is filled into an indirect heat exchanger, with theprills occupying about 80% of the volume of the heat exchanger. Theprills are then rehydrated by adding the theoretical amount of water tothe prills. Indirect heat exchange results in the expected reversableabsorption and release of heat over many cycles.

It is mentioned above that the particulate composition can be made by amethod comprising blending the storage material with partiallypolymerised material and it is also mentioned above that whenpolyacrylic acid is blended with calcium chloride there can beuncontrolled cross-linking and insolubilisation. We have found that itis possible to overcome difficulties that may occur during blending ofsome prepolymers with some storage materials during the formation ofparticulate compositions and that by the same technique it is possibleto overcome the difficulties that may occur in any method of forming athermal energy storage composition by blending a polymeric material witha thermal energy storage material that includes polyvalent metal ions.

In particular we believe that the problem has arisen from the fact thatmany of the polymers that have been used previously have been slightlyor highly anionic, for instance being polymers of acrylic acid orcopolymers of acrylamide and acrylic acid, and many of the storagematerials have included polyvalent cations such as calcium and that thishas caused uncontrolled cross-linking and insolubilisation of theanionic polymer.

According to a second aspect of the invention a cationic polymericmaterial is used. Thus a thermal energy storage composition according tothis aspect of the invention comprises a thermal energy storage materialthat includes polyvalent metal ions and that is held in a thermallyconducting matrix of cationic polymeric material.

The composition may be made by forming a blend of polymeric material andthe chosen storage material in any convenient manner. Thus a solutionmay be formed of cationic polymerisable material and this may bepolymerised while the storage material is homogeneously distributed init, for instance as a result of being partly or fully dissolved in thesolution. However the invention is of most value when the composition ismade by mixing together preformed cationic polymer, water and thestorage material containing polyvalent metal ions. For instance polymermay be added to a solution or dispersion of the storage material inwater or, preferably, the storage material may be added to a viscoussolution of the polymer in water.

The matrix may be the structure that results from this blending, butgenerally the blended aqueous system is subjected to furtherpolymerisation so as to provide the matrix with a higher viscosity or toconvert it to a solid. Thus the cationic polymer that is blended withstorage material may be polymerisable, for instance a blend of polymerand monomer, but preferably is a cross-linkable polymer that eitherincludes cross-linking agent or to which cross-linking agent can beadded either at the time of blending with the inorganic hydrate orsubsequently.

Any such cross-linking or further polymerisation may be conducted whilethe blend is enclosed within the heat exchange or other vessel in whichit is to be used or whilst it has any other convenient shape. Forinstance the final polymerisation may be conducted while the compositionis in the form of a slab, sheet, tubes or particles, for instanceobtained by comminution of a gel formed by blending storage materialwith an aqueous solution of the linear polymer that is to becross-linked or further polymerised.

The matrix in the final composition may be a viscous liquid, provided ithas sufficient thickening effect to maintain the desired homogeneousdistribution of the storage material, or may be a solid. The solid maybe a soft or flexible solid, thus having a texture such that it can bedescribed as a non-flowable gel, or may be a relatively rigid solid. Ifit is solid then thermal energy may be transmitted to or extracted fromthe storage material by direct contact of the composition with a heatexchange fluid that is inert to the composition, generally a non-aqueousheat exchange fluid. For instance heat exchange fluid may percolatethrough a bed of particles of the composition. Generally however thecomposition is used for indirect heat exchange, being formed or fittedin an indirect heat exchanger and used in conventional manner.

The thermal energy storage materials used in the invention may be any ofthose discussed above provided that the material includes at least onecomponent that is a compound of a polyvalent metal. Preferably at least20% and at least usually 50%, typically 80% or more, of the total weightof the thermal energy storage material is provided by the compound orcompounds of polyvalent metal. Nucleating agent may be included, asdiscussed above.

Typical polyvalent metal compounds that may be present in thermal energystorage materials, generally as part or all of the compound thatreversably undergoes a change of state, include calcium chloridehexahydrate, calcium chloride tetrahydrate, calcium nitratetetrahydrate, barium hydroxide octahydrate and zinc nitrate hexahydrate.

The polymer is generally a homopolymer or copolymer of one or morecationic monomers, optionally with other monomers, that preferably aresoluble in water and provide a linear polymer that is soluble in water.The monomers are preferably acrylic monomers. They may be cationicderivatives of acrylamide or methacrylamide for instance dialkyl aminoalkyl acrylamide, but preferably are cationic derivatives of acrylic ormethacrylic acid, for instance dialkylaminoalkyl esters of acrylic ormethacrylic acid. In any such dialkylaminoalkyl groups the alkyl andalkylene groups generally contain from 1 to 4 carbon atoms each, and arenormally methyl or ethyl groups. Preferably the dialkylamino compound isquaternised with any convenient quaternising agent, for instance methylchloride, methyl bromide or dimethyl sulphate. Preferred monomerstherefore are quaternised dimethyl or diethyl amino methyl or ethylacrylate or methacrylate. The polymer may be a Mannich base of apolyacrylamide.

Comonomers that may be copolymerised with such monomers include acrylicand methacrylic acid, acrylic and methacrylic esters and acrylic andmethacrylic amides. Copolymers of the described cationic groups withacrylamide or methacrylamide are preferred. If the polymer containsanionic groups the molar proportion of cationic groups must be higherthan the molar proportion of anionic groups. Generally the polymercontains from 10 to 100%, preferably 20 to 60% cationic groups with thebalance non-ionic groups, the percentages being on a molar basis.

Other suitable polymers include polymers made by reaction of methylamineand epichlorhydrin.

When the polymer is to be further polymerised after blending with thethermal energy storage material this polymerisation may follow from theinclusion of polymerisable acrylic monomer with the polymer, forinstance a cross-linking agent such as methylene bisacrylamide ormethylol acrylamide, or by reaction with some other cross-linking agent,for instance aldehydes such as glutaraldehyde or formaldehyde that willreact with free amide groups in the polymer.

The thermal conductivity and weight of the matrix may all be asdescribed above.

When the thermal energy storage material is a hydrate the compositionmay be formed with the desired amount of water for hydration in it, forinstance by combining calcium chloride, polymer and sufficient water toform the desired hexahydrate or other hydrate within the matrix.Alternatively the water content of the composition may be adjusted afterits formation, by evaporation or by absorption, for instance of thetheoretical amount of water for forming the hydrate.

The following is an example of this aspect of the invention.

EXAMPLE 3

5 grams of a copolymer of equal parts by weight (pbw) acrylamide andquaternised dimethylaminoethyl acrylate and 54 grams of a mixture of89.6 pbw anhydrous CaCl₂, 8.3 pbw KCl and 2.1 pbw SrCl₂ 6H₂ O areblended into 47 grams water. 0.7 grams formaldehyde are added as across-linking agent and the mixture is immediately filled into anindirect heat exchange vessel, polymerisation and cross-linkingoccurring within the vessel and converting the polymeric material into asolid polymeric matrix. The product is allowed to cool and the expectedcrystallisation of the hydrated salt, with accompanying release of heat,is observed. This transition can be reversably repeated over manycycles.

Even when the heat storage material does not cause cross-linking ofpolymeric material it may still be rather difficult to obtainhomogeneous blending of polymeric material and heat storage material.Another difficulty is that, as mentioned above, unless the compositionis in the form of free flowing particles it can be difficult to fill thecomposition into the heat exchange vessel.

According to a third aspect of the invention we provide a novel processfor making a thermal energy storage composition comprising a thermalenergy storage material held in a thermally conducting matrix ofpolymeric material formed by polymerisation of polymerisable material.This process comprises forming a solution in which all the polymerisablematerial and most or all of the storage material are dissolved andpolymerising the polymerisable material in this solution. In thepreferred method of the invention a low viscosity fluid comprising orconsisting of the solution is introduced into a vessel that is to serveas an indirect heat exchanger and the polymerisation is conducted in thevessel. The matrix is thus formed in situ in the vessel in which it isto be used.

The fluid must have a sufficiently low viscosity that it flows easilyinto conformity with the profile of the heat exchange vessel andnormally has a viscosity of less than 150 poise, generally less than 100poise, e.g. 10 to 50 poise, measured at 30° C.

The fluid may be a dispersion or emulsion in a liquid of the solution ofpolymerisable material. For instance an aqueous solution of monomer andstorage material may be emulsified or dispersed in an organic liquid.Preferably however the fluid consists mainly or only of the solution.

A preferred method comprises forming separate solutions of thepolymerisable material and the storage material and combining thesolutions. The solvent is preferably water. The solution formed uponmixing the solvent, water and polymerisable material should have a lowviscosity, e.g. as described above, in order to facilitate good mixing.

The polymerisable material generally consists of one or more monomersbut it may contain soluble polymer or prepolymer, for instance toincrease its viscosity sufficient to reduce separation of any solidmaterial present initially in the solution or that is formed during thepolymerisation. Generally however the polymerisation that occurs in thevessel should involve lengthening the polymer chains of most or all ofthe polymerisable material. It may also involve cross-linking with theresult that the final matrix comprises cross-linked polymer.

The polymeric material that provides the matrix of the composition maybe formed from a wide variety of polymerisable monomers, that may beintroduced either as monomer or as prepolymer. The preferred monomersare ethylenically unsaturated monomers, especially acrylic monomers. Ifthe thermal energy storage material is to be present as a hydrate it isdesirable that the matrix should be capable of absorbing some water.Suitable acrylic monomers include acrylic and methacrylic acid and theirsalts, acrylic and methacrylic esters and acrylic and methacrylic amidesand N-substituted amides and other monomers having sufficient solubilityin the polymerisation solvent (generally water), preferred monomersbeing acrylamide, sodium acrylate and dialkyl amino alkyl acrylates andquaternised derivatives thereof, for example quaterniseddimethylaminoethyl acrylate. Other monomers that may be homopolymerisedor copolymerised include vinyl pyrrolidinone and vinyl sulphonic acid.

Suitable cross-linking agents for use with acrylic monomers includemethylene bisacrylamide and methylol acrylamide and soluble condensationpolymers such as polyethylene glycol diesters.

The thermal conductivity and the amount of the matrix polymer may be asdiscussed above.

The matrix polymer, and thus the composition, may be a viscous liquidthat has a viscosity sufficiently high to hold the storage material in astable distribution but preferably it is a solid. It may be soft andflexible, for instance being a non-flowable gel, or it may be arelatively rigid solid. The thermal conductivity of the matrix, and theamount of polymer, may be as discussed above.

The thermal energy storage material may include a nucleating agent andmay be as discussed above. Preferably it is a hydrate, as discussedabove, and preferred materials include sodium sulphate, calcium chlorideand other inorganic hydrates as discussed in the aforementioned Britishand U.S. Patent Specifications.

The polymerisable material and at least some of the storage material aredissolved in the water or other chosen solvent and the mixture subjectedto polymerisation. Usually at least 50%, and preferably at least 75%, byweight of the storage material (and preferably all the storage materialapart from any insoluble nucleating agent) is dissolved in the initialsolution. If the fluid mixture contains any undissolved storage material(e.g. nucleating agent) the fluid should be a homogeneous dispersion ofthe solid phase in the fluid mixture, and it may be necessary to thickenthe system (e.g. by including prepolymer) or to agitate the system inorder to minimise phase separation until polymerisation has occurred. Ifany undissolved material is present its amount should be less than 50%and preferably less than 20% of the total weight of storage material.

Polymerisation may be by thermal or redox systems, including thermallydecomposing initiators such as ammonium persulphate.

The vessel in which the system is polymerised may be of any suitableshape and construction for use as an indirect heat exchanger.

When the storage material is a hydrate polymerisation is generallyconducted in the presence of sufficient water to form the desiredhydrate. Thus for instance sodium sulphate decahydrate or calciumchloride hexahydrate may be provided by combining the anhydrous salt ina polymerisable mixture containing sufficient water to form the desiredhydrate within the matrix. The water content may be adjusted afterpolymerisation by evaporation or by absorption provided the matrix iswater permeable.

The following are some examples of this third aspect of the invention.

EXAMPLE 4

In this example a matrix is formed in an indirect heat exchange vesselof a copolymer of acrylamide and dimethylsulphate quaterniseddimethylaminoethyl acrylate (DMSqDMAEA). The matrix is formed by gelpolymerisation using a monomer salt mixture including redox initiator,as follows:

    ______________________________________                                        CaCl.sub.2           48.4 g                                                   KCl                   4.5 g                                                   SrCl.sub.2 6H.sub.2 O                                                                               1.0 g                                                   Acrylamide soln (52.5%)                                                                             7.6 g                                                   DMSqDMAEA soln (68.0%)                                                                              5.9 g                                                   H.sub.2 O            41.6 g                                                   ______________________________________                                    

The mixture is added to the vessel at a temperature of 38° C. andinitiated by adding Na₂ SO₃ and KBrO₃ and rotated end over end to keepthe solid material dispersed. A temperature rise to 60° C. is seen andafter 10 minutes the vessel contents are found to exist as a rubberygel. When a cool heat exchange liquid effects indirect heat exchangewith the gel in the vessel crystallisation of the calcium chloride andheating of the heat exchange liquid is observed. The transition can bereversably repeated over many cycles without any deterioration of thesample.

EXAMPLE 5

In this example a gel composition is formed in which the polymer iscross-linked polyacrylamide by gel polymerisation using thermalinitiation. A monomer/salt mixture is formed as follows:

    ______________________________________                                        CaCl.sub.2             48.4   g                                               K Cl                   4.5    g                                               SrCl.sub.2 6H.sub.2 O  1.0    g                                               H.sub.2 O              39.9   g                                               Acrylamide soln (52.5%)                                                                              15.2   g                                               Methylene bisacrylamide                                                                              0.016  g                                               Ammonium persulphate   0.004  g                                               ______________________________________                                    

This mixture is introduced into an indirect heat exchange vessel andthermally polymerised with agitation at 60° to 90° C. Indirect heatexchange can be conducted satisfactorily over many cycles.

We claim:
 1. A thermal energy storage composition comprising a thermalenergy storage hydrate material that includes polyvalent metal ions andthat is held in a thermally conducting matrix of cationic polymericmaterial that is water absorbent.
 2. A composition according to claim 1in which the thermal energy storage material comprises an inorganichydrate that will undergo a change of hydration, with accompanyingrelease or storage of latent heat of hydration, at a temperature of 10°to 100° C.
 3. A composition according to claim 1 in which the polymericmaterial is a cationic acrylic polymer.
 4. A composition according toclaim 1 in which the polymeric material is selected from optionallyquaternised polymers and copolymers of dialkylaminoalkyl(meth) acrylate,and optionally quaternised polymers and copolymers ofdialkylaminoalkyl(meth) acrylamide.
 5. In a method of making a thermalenergy storage composition comprising a thermal energy storage materialthat includes polyvalent metal ions and that is held in a thermallyconducting matrix of polymeric material by a method comprising blendingthe storage material with ionic polymeric material to form the matrixand water, the improvement consisting of using cationic polymericmaterial as the ionic material.
 6. In the method according to claim 5 inwhich the thermal energy storage material comprises an inorganic hydratethat will undergo a change of hydration, with accompanying release orstorage of latent heat of hydration, at a temperature of 10° to 100° C.7. In the method according to claim 5 in which the polymeric material isa cationic acrylic polymer.
 8. In the method according to claim 5 inwhich the polymeric material is selected from optionally quaternisedpolymers and copolymers of dialkylamino-alkyl(meth) acrylate andoptionally quaternised polymers and copolymers ofdialkylaminoalkyl(meth) acrylamide.
 9. In the method according to claim5 in which the polymeric material in the resulting blend iscross-linked.
 10. In the method according to claim 5 in which saidblending is carried out by blending the storage material with an aqueoussolution of the polymeric material.